Calendar change of 1752

In 1752 Britain skipped eleven days: Wednesday 2 September was followed by Thursday 14 September.
Day off

Writing a list of things to do on a day off turns it into a day on; physicists call this the Errand Effect.
Day after tomorrow

The day after tomorrow moves away at exactly one day per day, so it has been two days away since records began.
Weekend

The weekend is two days long and lasts about forty minutes, a discrepancy that has never been fixed.
Leap day

Leap day is a spare day kept in storage and added to February every four years, when February isn’t looking.
Tomorrow

Tomorrow has never been reached; every expedition to it has arrived at today and been asked to wait.
Saturday
Saturday is 24 hours long, officially; the public has measured it at nine hours and forty minutes.
Friday

Friday can be detected as early as Tuesday afternoon, usually by whoever sits nearest the window.
Sunday

Sunday is the first or last day of the week; its committee meets every Sunday, always too early or too late.
Monday

Monday arrives every week whether or not anyone asks; attempts to skip it have so far resulted in Monday.